In production of sheet, strips and other steel products it is necessary to remove surface defects like cracks, flakes and scabs, before the slabs or billets are rolled out. The surface defects are removed by thermochemical scarfing, in which strong heat and oxygen are acting in the surface of the slab or billet. In thermochemical scarfing the scarfed surface is melted down and a certain part of the molten iron is oxidized, a considerable amount of heat being developed simultaneously.
In practice either so called surface scarfing or spot scarfing is applied. In surface scarfing the entire surface of the slab or billet is scarfed to a certain depth, determined by quality claims, or to the penetration of the deepest defect occurring. This method is obviously uneconomic as, in some cases, it is necessary to remove up to 2 to 4% of the total weight of the slab or billet. For that reason, especially in cold scarfing, so called spot scarfing is often used. Scarf scarfing implies local removal of surface defects. The most common method is selective manual scarfing, but lately efforts have also been made to design machines for selective spot scarfing. The great problem as to selective spot scarfing is the ignition of the exothermic reaction which makes thermochemical scarfing possible.
A scarfing process with sufficiently rapid ignition is a necessary condition to realize a fast working selective spot scarfer, as flying start must be used.
The best known ignition method is to feed an iron wire or iron powder into the flame during the ignition period. Generally fuel gas flames (e.g., acetylene with oxygen) are used which are directed towards the point of the work-piece where the scarfing is to start. The iron wire or iron powder is melted and oxidized in the hot fuel gas flame and the stream of scarfing oxygen. The hot slag and the heat from the combustion of the iron wire or iron powder together with the heat from the fuel gas flame hit the work-piece and ignite the scarfing process. The use of iron wire or iron powder shortens the ignition time considerably. In spite of that the ignition takes at best from 2 to 4 seconds. Even if the work-piece is moving at relatively low velocity, e.g., 5 cm/s, its movement during the ignition period will still be unacceptably long. Consequently, either the work-piece must be stopped or the torch move together with the work-piece during the ignition. If the work-piece must be stopped, there will, of course, be no flying start, and if the torch must move together with the work-piece, such a design will be complicated and less reliable. The use of iron or iron powder as filler material for ignition which is needed in relatively large quantities, sometimes causes a larger cost for the consumed filler material than the cost of the iron saved by the selective spot scarfing.
It has been suggested that the ignition of the spot scarfing process could be shortened further by means of a method using an electric arc for the ignition process. The arc is burning between a non-consumable pilot electrode and the region of the work-piece where the scarfing is to start. Immediately after a molten pool has appeared in the desired region of the work-piece, a stream of scarfing oxygen together with an oxyacetylene flame from a conventional scarfing torch is directed toward the molten pool, and the electric pilot electrode is rapidly withdrawn from the operating region. It is said that the ignition time with this method can be shortened further, 10 to 20 times, compared with a conventional ignition without pilot electrode or iron wire. This ignition takes 5-10 seconds. Thus it would be possible to bring down the ignition period to some tenths of a second. A serious disadvantage with this method is that the pilot electrode which strikes in an active medium is exposed to considerable wear and spatter of slag from the molten pool.
Another method has been proposed to initiate the combustion process in spot scarfing with flying start, namely to use a consumable electrode of iron or of other metal. An electric arc between the electrode tip and the billet causes a part of the wire to be consumed and together with the molten metal in the surface of the billet forms the starting point of the thermochemical combustion process in the surface of the billet during the scarfing process. The initiation of the chemical combustion as well as the scarfing is carried out with a torch, comprising oxygen nozzles and gas flames in a conventional manner. However, it seems to be necessary in this case that the electric arc for a certain time be burning in the direction of one and the same point in the surface of the billet to make ignition possible. To attain a flying start it is necessary that the electrode holder, by means of a mechanical arrangement, make such a movement in relation to the torch head, passing over the billet, that said holder in relation to the surface of the billet is at rest while the arc is burning. The implementation of such a mechanical arrangement is extremely difficult.
In all the methods, mentioned above and used hitherto, the combustion of iron in the scarfing process is not an entirely self-supporting reaction, as to the need for heat. Heat must be supplied which, as mentioned above, is usually done with one or more fuel gas flames, fed for instance with acetylene and oxygen, which are directed toward the operating region. Although this is the most common scarfing method in practice, it still has disadvantages.
The disadvantages are inherent both in principle and design. The scarfing device is also used to start the chemical combustion. As it is designed for maximum scarfing rate, it cannot fulfill the requirements of a device for starting the chemical combustion process in an optimal way. Gases from the fuel gas flames mix with the scarfing oxygen and rarefy it which has the effect that the start of the chemical combustion is made still more difficult and that the scarfing rate is limited. The streams of oxygen and hot fuel gases even influence each other so that starting pits, ridges and burrs are easily formed in the scarfed surface and the process is hard to control. The poor controlability of the process results in the problem that, while scarfing is in progress, the scarfing depth can be changed only by regulation of the scarfing speed. In ramp construction of spot scarfers with several independently working torches; there might be -- among other things -- difficulties in keeping the low heat flames burning in the torches and they might not work for a moment.
Another disadvantage with fuel gas flames for maintenance of the combustion process is the risk of explosion and backfire in the torches.